Recently, various display and lighting devices have been under active study and development, particularly those based on electroluminescence (EL) from organic materials.
Many organic materials exhibit fluorescence (i.e., luminescence from a symmetry-allowed process) from singlet excitons. Since this process occurs between states of equal symmetry, it may be very efficient. On the contrary, if the symmetry of an exciton is different from that of the ground state, then the radioactive relaxation of the exciton is disallowed and luminescence will be slow and inefficient. Because the ground state is usually anti-symmetric, the decay from a triplet breaks the symmetry. The process is thus disallowed and the efficiency of EL is very low. Therefore, the energy contained in the triplet state is mostly wasted.
The luminescence from a symmetry-disallowed process is known as phosphorescence. Characteristically, phosphorescence may persist up to several seconds after excitation due to the low probability of the transition, in contrast to fluorescence which shows rapid decay. The use of phosphorescent materials has been a major breakthrough in boosting electroluminescence efficiency because they allow for the simultaneous harvesting of both singlet and triplet excitons. Selecting a suitable host material for the phosphorophore dopants remains one of the critical issues in phosphorescence-based OLEDs. The host material is important because efficient exothermic energy transfer from the host material to the dopant phosphorophore depends on whether the triplet-state energy of the host is greater than that of the dopant.
Well known host materials for guest-host systems include hole-transporting 4,4′-N,N′-dicarbazol-biphenyl (CBP) and electron-transporting aluminum 8-hydroxyquinoline (AlQ3), which have both been used in OLEDs. However, the known host materials are not suitable for all phosphorescent guests. For example, the host compound for phosphorescent emitters must fulfil an important condition that the triplet energy of the host shall be higher than that of the phosphorescent emitter. In order to provide efficient phosphorescence from the phosphorescent emitter, the lowest excited triplet state of the host has to be higher in energy than the lowest emitting state of the phosphorescent emitter. Since emission from the phosphorescent emitter is desired, the lowest excited state has to be from the phosphorescent emitter, not the host compound. As such, there continues to be a need in the art for suitable host materials for guests which have short emission wavelengths in the light spectrum, e.g., in the blue region of the spectrum.
Several host materials for better phosphorescent emission have been reported. Due to their charge conducting ability, photophysical and redox properties, sufficiently large triplet energies and carrier-transport properties, carbazole-based compounds have been actively studied.
For example, U.S. Patent Application Publication No. US 2003/205696 assigned to Canon KK discloses guest-host emissive systems suitable for use with organic light emitting devices in which the host material comprises a compound having a carbazole core with an electron-donating species bonded to nitrogen, aromatic amine groups or carbazole groups bonded to one or more of the carbon atoms, a large band gap potential, and high-energy triplet excited states. Such materials permit short-wavelength phosphorescent emission by an associated guest material, and the combination of said materials with emissive phosphorescent organometallic compounds such as platinum complexes is useful in the fabrication of organic light emitting devices.
Li et al., “Novel fluorene/carbazole hybrids with steric bulk as host materials for blue organic electrophosphorescent devices,” Tetrahedron, 63(41):10161-10168 (2007) discloses the use of sterically hindered spacers in phosphorescent dopants to prevent or reduce the problem of self-quenching in organic electrophosphorescence devices. Novel fluorene/carbazole hybrids with tert-butyl substitutions, namely 9,9-bis[4-(3,6-di-tert-butylcarbazol-9-yl)phenyl]fluorene (TBCPF) and 9,9-bis[4-(carbazol-9-yl)phenyl]-2,7-di-tert-butylfluorene (CPTBF), reportedly exhibit not only high triplet energy (>2.8 eV) but also high glass transition temperature (Tg) (>160° C.) and thermal stability.
Further, Wu et al., “The Quest for High-Performance Host Materials for Electrophosphorescent Blue Dopants,” Adv. Funct. Mater., 17: 1887-1895 (2007) discloses 3,5-di(N-carbazolyl)tetraphenylsilane (SimCP) and N,N′-dicarbazolyl-3,5-benzene (mCP) as host materials for phosphorescent blue dopants, while Thoms et al., “Improved host material design for phosphorescent guest-host systems,” Thin Solid Films 436: 264-268 (2003) discloses a series of carbazole-based compounds as host materials in an iridium phosphor-based guest-host organic light emitting diode and the results of semi-empirical calculations.
However, none of the above-disclosed materials meet all the requirements necessary for OLED application, e.g., suitable energy level, charge transport ability, processability from a solution with uniform film formation, ability to form an amorphous phase, ability for good dopant dispersion, morphological stability (high Tg), thermal and electrochemical stabilities under operational conditions of the device. Therefore, there has been a need to develop new host materials which are capable of satisfying all of the requirements indicated above.